1. Field of the Invention
This invention relates generally to cutting elements for rotary drill bits. Specifically, this invention relates to the mounting of polycrystalline diamond compact (PDC) cutting elements to carrier elements to form a cutting structure in a fashion to reduce concentration of stress at selected interfaces in the carrier/cutting element substructure, which stress concentrations otherwise lead to premature degradation and destruction of the cutting structure and bonds therein.
2. State of the Art
Cutting elements used in rotary drill bits generally comprise a relatively thin layer or table of polycrystalline diamond formed under ultrahigh temperature and pressure on a less hard substrate, typically made of cemented tungsten carbide (WC). The tungsten carbide substrates are then attached by brazing to carrier elements or "back-ups" (also typically of WC) which are secured to the crown of a drill bit, which in turn is connected to a drill string and lowered into a hole for rotary drilling.
PDC cutting elements are usually either round or semicircular. Wear on the PDC cutting element is greatest along the outermost arcuate edge of the element which is in contact with the rock formation, while the remainder of the cutting element remains in a relatively pristine state. Thus, uneven wear and degradation in only one portion of the PDC cutting element necessitates disposing of a greater percentage of relatively unused polycrystalline diamond. Therefore, the industry from time to time has focused on using semicircular PDC elements (hereinafter "segmented" PDC cutting elements) which reduce the amount of wasted polycrystalline diamond material. U.S. Pat. No. 4,498,549, issued to Jurgens and assigned to the assignee of the present invention and U.S. Pat. No. 4,767,050, issued to Flood et al., disclose cutting structures including segmented PDC cutting elements.
Experience has shown that use of semicircular or segmented PDC cutting elements bonded to carrier elements creates stress concentrations at various points or along lines in the cutting structure which are not observed when circular PDC cutting elements are mounted to carrier elements. Stress concentrations occur most often at points or lines of bonding between the cutting element substrate, the tungsten carbide carrier element and a blank, which may also be of WC, mounted adjacent the PDC cutting element on the carrier element to form a wear surface substantially coextensive with the cutting element PDC surface. Alternatively, a shelf or shoulder may be created on the carrier element itself in lieu of the use of the aforementioned wear surface blank, as disclosed in the aforementioned Flood patent, but this type of structure also experiences stress concentrations along the inner edge of the carrier element shelf. With either type of assembly, such stress concentrations promote fracture of the carrier element behind the PDC cutting element, and may also contribute to delamination of the cutting structure as well as spalling and fracture of the PDC cutting element. As a result of such damage, the rate of penetration of the drill bit will be severely reduced, necessitating a halt to drilling and retrieval of the bit for repair or replacement. Stress-related cutting structure failure is particularly a problem with larger cutting elements, for example 3/4" diameter and larger, as the stresses are magnified by the size of the cutting structure and fewer cutters are employed on large-cutter bits, so that the loss of a single cutter may effectively bring drilling to a halt.
Cutting structure configurations which have been specifically designed to limit stress concentrations in assemblies of round or circular PDC cutting elements and carrier elements are disclosed in U.S. Pat. No. 4,993,505 to Packer et al. and U.S. Pat. No. 5,060,739 to Griffin, both of which patents address the occurrence of stress fractures at the bonding point between the tungsten carbide cutting element substrate and the stud (carrier element). Additionally, U.S. Pat. No. 5,061,293 to Barr et al. discloses a cutting element where a PDC layer is formed between a first layer and a second layer of tungsten carbide to stabilize the PDC layer during cutting.
None of the foregoing patents, however, disclose a means or method to minimize stress concentrations when a conventional, segmented PDC cutting element is bonded to a carrier element adjacent a blank or into a pocket, shelf, shoulder or other recessed structure of a carrier element. Thus, it would be advantageous to provide a cutting structure which relieves stress forces at the interface between a segmented PDC cutting element and the stud or other carrier element of the cutting structure.